Tubing made of zirconium alloys is widely employed in the nuclear industry, primarily as cladding for nuclear fuel rods. This particular application requires a relatively thin-walled tubing that is resistant both to chemical attack and mechanical attack. Such tubing is typically formed by a combination of mechanical and thermal treatments. Pilgering, one commonly employed tubing formation method, changes the texture of the tubing by gradually reducing the cross-sectional diameter of the tubing while simultaneously increasing the axial elongation of the tubing until the desired optimum final diameter and tube wall thickness are achieved. This process causes the hydrides in the tubing material to be oriented in a circumferential direction.
Nuclear fuel cladding tubes subjected to nuclear radiation for the prolonged periods of time characteristic of nuclear reactor operating cycles tend to expand axially. This irradiation-induced axial cladding deformation is also accompanied by a reduction in the radial thickness of the tubing wall and a concomitant decrease in end of cycle life ductility. Texturing the cladding tubes has been found to reduce or avoid some of the problems associated with lengthy use in a reactor environment. One method of texturing, which reduces axial irradiation growth and increases end of cycle life ductility, has been proposed in U.S. Pat. No. 3,804,708. This result is achieved by expanding the tube diameter while constraining the tube ends to prevent an increase in length relatively greater than the increase in diameter. Although tubing having the texture produced according to this method represents an improvement over that of previously available tubing, it is not as resistant to pellet-cladding interaction as could be desired.
The zirconium alloys preferred for use in tubing for nuclear applications have a hexagonal close packed crystal structure. The orientation of the basal poles of the metal crystals has been determined to have a significant effect on the texture and, hence, the ultimate properties of tubing formed from zirconium alloys. Increasing the zirconium alloy radial texture produces tubing that is less likely to be susceptible to chemical and/or mechanical attack than nontextured or only slightly textured tubing metal. The method of the aforementioned U.S. Pat. No. 3,804,708 orients the tubing zirconium alloy crystals so that the basal pole principal components are predominantly in both the radial and axial directions. However, orientation of the basal poles to the radial direction to increase the radial texture produces tubing with enhanced properties.
A method of increasing the radial texture of basal poles in the crystal structure of zirconium is disclosed in U.S. Pat. No. 4,765,174. The method described in this patent, however, only achieves an appreciable increase in final tube radial texture when the tubing is processed during the intermediate stages prior to expansion to the final tubing diameter. The zirconium alloy tubing processing method described in this patent exhibits an improved texture over that previously achieved. However, the split radial texture characteristic of zirconium alloy tubing produced according to this method may not provide optimal pellet-cladding-interaction resistance or resistance to chemical attack and/or mechanical deformation. Moreover, because the thermal and mechanical processing described in U.S. Pat. No. 4,765,174 takes place before the final tubing expansion, the degree of precise control over the final tubing texture desired may not always be possible.
A need exists, therefore, for a method of texturizing zirconium alloy tubing for use as cladding in nuclear fuel rods that is performed during the final tubing processing stages to produce tubing with an increased radial texture. The prior art fails to disclose a method for producing a zirconium alloy tubing characterized by a high degree to radial texture wherein the increased texture is produced during the final stage of tubing fabrication. Further, the prior art fails to disclose a method for producing a highly textured zirconium alloy tubing having a single peak radial basal pole texture.